Internet Engineering Task Force S. Aldrin
Internet-Draft Google
Intended status: Informational C. Pignataro, Ed.
Expires: March 11, 2018 N. Kumar, Ed.
Cisco
N. Akiya
Big Switch Networks
R. Krishnan
A. Ghanwani
Dell
September 7, 2017
Service Function Chaining (SFC)Operation, Administration and Maintenance (OAM) Frameworkdraft-ietf-sfc-oam-framework-03
Abstract
This document provides a reference framework for Operations,
Administration and Maintenance (OAM) for Service Function Chaining
(SFC).
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 11, 2018.
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Internet-Draft SFC OAM Framework September 20172. SFC Layering Model
Multiple layers come into play for implementing the SFC. These
include the service layer and the underlying layers (Network, Link
etc)
o The service layer in Figure 1, consists of SFC data plane elements
that includes classifiers, Service Functions (SF), Service
Function Forwarders (SFF), SFC Proxy. This layer uses the overlay
network for ensuring connectivity between SFC data plane elements.
o The overlay network layer in Figure 1, leverages various overlay
network technologies interconnecting SFC data plane elements and
allows establishing service function paths (SFPs). This layer is
mostly transparent to the SFC data plane elements.
o The underlay network layer in Figure 1, is dictated by the
networking technology deployed within a network (e.g., IP, MPLS)
o The link layer in Figure 1, is dependent upon the physical
technology used. Ethernet is a popular choice for this layer, but
other alternatives are deployed (e.g. POS, DWDM etc...). The
same or distinct link layer technologies may be used in each leg
shown in figure 1.
o----------------------Service Layer----------------------o
+------+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
|Classi|---|SF1|---|SF2|---|SF3|---|SF4|---|SF5|---|SF6|---|SF7|
|fier | +---+ +---+ +---+ +---+ +---+ +---+ +---+
+------+
o------VM1------o o--VM2--o o--VM3--o
o-----------------o-------------------o---------------o Overlay network
o-----------------o-----------------------------------o Underlay network
o--------o--------o--------o--------o--------o--------o Link
Figure 1: SFC Layering Example
While Figure 1 depicts a sample example where SFs are enabled as
virtual entities, the SFC architecture does not make any assumptions
on how SFC data plane elements are deployed. The SFC architecture is
flexible to accomodate physical or virtual entity deployment. SFC
OAM adheres to this flexibility and accordingly it is applicable
whether SFC data plane elements are deployed directly on physical
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hardware, as one or more Virtual Machines, or any combination
thereof.
3. SFC OAM Components
The SFC operates at the service layer. For the purpose of defining
the OAM framework, the service layer is broken up into three distinct
components.
1. SF component: OAM solutions for this component include testing
the service functions from any SFC-aware network devices (i.e.
classifiers, controllers, other service nodes).
2. SFC component: OAM solutions for this component include testing
the service function chains and the SFPs, validate the
correlation between a Service Function Chain and the actual
forwarding path followed by a packet matching that SFC, etc.
3. Classifier component: OAM solutions for this component include
testing the validity of the classification rules and detecting
any incoherence among the rules installed in different
classifiers.
Below figure illustrates an example where OAM for the three defined
components are used within the SFC environment.
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+-Classifier +-Service Function Chain OAM
| OAM |
| | ______________________________________________
| \ /\ Service Function Chain \
| \ / \ +---+ +---+ +-----+ +---+ \
| \ / \ |SF1| |SF2| |Proxy|--|SF3| \
| +------+ \/ \ +---+ +---+ +-----+ +---+ \
+----> | |...(+-> ) | | | )
|Classi| \ / +-----+ +-----+ +-----+ /
|fier | \ / | SFF1|----| SFF2|----| SFF3| /
| | \ / +--^--+ +--^--+ +-----+ /
+----|-+ \/____________|________________________________/
| |
+----------SF_OAM------+
+---+ +---+
+SF_OAM>|SF3| |SF5|
| +-^-+ +-^-+
+------|---+ | |
|Controller| +-SF_OAM+
+----------+
Service Function OAM (SF_OAM)
Figure 2: SFC OAM for Three Components
It is expected that multiple SFC OAM solutions will be defined, many
targeting one specific component of the service layer. However, it
is critical that SFC OAM solutions together provide the coverage of
all three SFC OAM components: the service function component, the
service function chain component and the classifier component.
3.1. Service Function Component3.1.1. Service Function Availability
One SFC OAM requirement for the service function component is to
allow an SFC-aware network device to check the availability to a
specific service function, located on the same or different network
devices. Service function availability is an aspect which raises an
interesting question. How to determine that a service function is
available?. On one end of the spectrum, one might argue that a
service function is sufficiently available if the service node
(physical or virtual) hosting the service function is available and
is functional. On the other end of the spectrum, one might argue
that the service function availability can only be concluded if the
packet, after passing through the service function, was examined and
verified that the packet got expected service applied.
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The former approach will likely not provide sufficient confidence to
the actual service function availability, i.e. a service node and a
service function are two different entities. The latter approach is
capable of providing an extensive verification, but comes with a
cost. Some service functions make direct modifications to packets,
while other service functions do not make any modifications to
packets. Additionally, purpose of some service functions is to,
conditionally, drop packets intentionally. In such case, packets
will not be coming out from the service function. The fact is that
there are many flavors of service functions available, and many more
flavors of service functions will likely be introduced in future.
Even a given service function may introduce a new functionality
within a service function (e.g., a new signature in a firewall). The
cost of this approach is that verifier functions will need to be
continuously modified to "keep up" with new services coming out: lack
of extendibility.
This framework document provides a RECOMMENDED architectural model
where generalized approach is taken to verify that a service function
is sufficiently available. More specifics on the mechanism to
characterize SF-specific OAM to validate the service offering is
outside the scope of this document. Those mechanism are
implementation and deployment specific.
3.1.2. Service Function Performance Measurement
Second SFC OAM requirement for the service function component is to
allow an SFC aware network device to check the loss and delay induced
by a specific service function. TBD - details will be provided in a
later revision.
3.2. Service Function Chain Component3.2.1. Service Function Chain Availability
Verifying an SFC is a complicated process as the SFC could be
comprised of varying SF's. Thus, SFC requires the OAM layer to
perform validation and verification of SF's within an SFP, as well as
connectivity and fault isolation.
In order to perform service connectivity verification of an SFC, the
OAM could be initiated from any SFC aware network devices for end-to-
end paths or partial path terminating on a specific SF within the
SFC. The goal of this OAM function is to ensure the SF's chained
together has connectivity as it is intended to when SFC was
established. Necessary return code should be defined to be sent back
in the response to OAM packet, in order to qualify the verification.
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When ECMP is in use at the service layer for any given SFC, there
must be the ability to discover and traverse all available paths.
TBD - further details will be provided in a later revision.
3.2.2. Service Function Chain Performance Measurement
Any SFC-aware network device must have the ability to perform loss
and delay measurements over the service function chain as a unit
(i.e. end-to-end) or to a specific segment of service function
through the SFC.
3.3. Classifier Component
A classifier maintains the classification rules that maps a flow to a
specific SFC. It is vital that the classifier is correctly
configured with updated classification rules and functioning
accordingly. The SFC OAM must be able to validate the classification
rules by assessing whether a flow is appropriately mapped to the
relevant SFC. Sample OAM packets can be presented to the classifiers
to assess the behavior with regards to a given classification entry.
4. SFC OAM FunctionsSection 3 describes SFC OAM operations that is required on each SFC
component. This section explores the same from the OAM functionality
point of view, which many will be applicable to multiple SFC
components.
Various SFC OAM requirements listed in Section 3, provides the need
for various OAM functions at different layers. Many of the OAM
functions at different layers are already defined and in existence.
In order to apply such OAM functions at service layer, they have to
be enhanced to operate a single SF/SFF to multiple SFs/SFFs in an SFC
and also in multiple SFCs.
4.1. Connectivity Functions
Connectivity is mainly an on-demand function to verify that the
connectivity exists between network elements and the availability
exists to service functions. Ping is a common tool used to perform
this function. OAM messages SHOULD be encapsulated with necessary
SFC header and with OAM markings when testing the service function
chain component. OAM messages MAY be encapsulated with necessary SFC
header and with OAM markings when testing the service function
component. Some of the OAM functions performed by connectivity
functions are as follows:
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o Verify the Path MTU from a source to the destination SF or through
the SFC. This requires the ability for OAM packet to take
variable length packet size.
o Verify the packet re-ordering and corruption.
o Verify the policy of an SFC or SF using OAM packet.
o Verification and validating forwarding paths.
o Proactively test alternate or protected paths to ensure
reliability of network configurations.
4.2. Continuity Functions
Continuity is a model where OAM messages are sent periodically to
validate or verify the reachability to a given SF or through a given
SFC. This allows monitor network device to quickly detect failures
like link failures, network failures, service function outages or
service function chain outages. BFD is one such function which helps
in detecting failures quickly. OAM functions supported by continuity
check are as follows:
o Ability to provision continuity check to a given SF or through a
given SFC.
o Notifying the failure upon failure detection for other OAM
functions to take appropriate action.
4.3. Trace Functions
Tracing is an important OAM function that allows the operation to
trigger an action (e.g., response generation) from every transit
device (e.g., SFF, SF, SFC Proxy etc) on the tested layer. This
function is typically useful to gather information from every transit
devices or to isolate the failure point towards an SF or through an
SFC. Some of the OAM functions supported by trace functions are:
o Ability to trigger action from every transit device on the tested
layer towards an SF or through an SFC, using TTL or other means.
o Ability to trigger every transit device to generate response with
OAM code(s) on the tested layer towards an SF or through an SFC,
using TTL or other means.
o Ability to discover and traverse ECMP paths within an SFC.
o Ability to skip un-supported SFs while tracing SFs in an SFC.
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Internet-Draft SFC OAM Framework September 20174.4. Performance Measurement Function
Performance management functions involve measuring of packet loss,
delay, delay variance, etc. These measurements could be measured
pro-actively and on-demand.
SFC OAM framework should provide the ability to perform packet loss
for an SFC. Measuring packet loss is very important function. Using
on-demand function, the packet loss could be measured using
statistical means. Using OAM packets, the approximation of packet
loss for a given SFC could be measured.
Delay within an SFC could be measured from the time it takes for a
packet to traverse the SFC from ingress SFC node to egress SFF. As
the SFCs are generally unidirectional in nature, measurement of one-
way delay [RFC7679] is important. In order to measure one-way delay,
time synchronization must be supported by means of NTP, PTP, GPS,
etc.
One-way delay variation [RFC3393] could also be measured by sending
OAM packets and measuring the jitter between the packets passing
through an SFC.
Some of the OAM functions supported by the performance measurement
functions are:
o Ability to measure the packet processing delay induced by a
service function or the one-way delay to traverse a service
function path along an SFC.
o Ability to measure the packet loss [RFC7680] within a service
function or a service function path bound to a given SFC.
5. Gap Analysis
This section identifies various OAM functions available at different
levels. It also identifies various gaps, if not all, existing within
the existing toolset, to perform OAM function required for SFC.
5.1. Existing OAM Functions
There are various OAM tool sets available to perform OAM functions
within various layers. These OAM functions could validate some of
the underlay and overlay networks. Tools like ping and trace are in
existence to perform connectivity check and tracing intermediate hops
in a network. These tools support different network types like IP,
MPLS, TRILL etc. There is also an effort to extend the tool set to
provide connectivity and continuity checks within overlay networks.
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Internet-Draft SFC OAM Framework September 20176. SFC OAM Model
This section describes the operational aspects of SFC OAM at the
Service layer to perform the SFC OAM function defined in Section 4
and analyze the applicability of various existing OAM toolsets in the
service layer.
6.1. SFC OAM Packet Marker
SFC OAM function described in Section 4 performed at the service
layer or overlay network layer must mark the packet as OAM packet so
that relevant nodes can differentiate an OAM packet from data
packets. The base header defined in Section 3.2 of
[I-D.ietf-sfc-nsh] assigns a bit to indicate OAM packets. When NSH
encapsulation is used at the service layer, the O bit must be set to
differentiate the OAM packet. Any other overlay encapsulations used
in future must have a way to mark the packet as OAM packet.
6.2. OAM Packet Processing and Forwarding Semantic
Upon receiving OAM packet, an SFC-aware SFs may choose to discard the
packet if it does not support OAM functionality or if the local
policy prevent it from processing OAM packet. When SF supports OAM
functionality, it is desired to process the packet and respond back
accordingly that helps with end-to-end verification. To avoid
hitting any performance impact, SFC-aware SFs can rate limit the
number of OAM packets processed.
Service Function Forwarder (SFF) may choose not to forward the OAM
packet to an SF if the SF does not support OAM function or if the
policy does not allow to forward OAM packet to an SF. SFF may choose
to skip the SF, modify the header and forward to next SFC node in the
chain. Although, skipping an SF might have implication on some OAM
function (e.g., delay measurement may not be accurate). How SFF
detects if the connected SF supports or allowed to process OAM packet
is outside the scope of this document. It could be a configuration
parameter instructed by the controller or can be a dynamic
negotiation between SF and SFF.
If the SFF receiving the OAM packet bound to a given SFC is the last
SFF in the chain, it must send a relevant response to the initiator
of the OAM packet. Depending on the type of OAM solution and tool
set used, the response could be a simple response (ICMP reply or BFD
reply packet) or could include additional data from the received OAM
packet (like stats data consolidated along the path). The proposed
solution should detail it further.
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Any SFC-aware node that initiates OAM packet must set the OAM marker
in the overlay encapsulation.
6.3. OAM Function Types
As described in Section 4, there are different OAM functions that may
require different OAM solutions. While the presence of OAM marker in
the overlay header (e.g., O bit in the NSH header) indicates it as
OAM packet, it is not sufficient to indicate what OAM function the
packet is intended for. The Next Protocol field in NSH header may be
used to indicate what OAM function is it intended to or what toolset
is used.
6.4. OAM Toolset applicability
As described in Section 5.1, there are different tool sets available
to perform OAM functions at different layers. This section describes
the applicability of some of the available toolsets in the service
layer.
6.4.1. ICMP Applicability
[RFC0792] and [RFC4443] describes the use of ICMP in IPv4 and IPv6
network respectively. It explains how ICMP messages can be used to
test the network reachability between different end points and
perform basic network diagnostics.
ICMP could be leveraged for basic OAM functions like SF availability
or SFC availability. The Initiator can generate ICMP echo request
message and control the service layer encapsulation header to get the
response from relevant node. For example, a classifier initiating
OAM can generate ICMP echo request message, can set the TTL field in
NSH header to 255 to get the response from last SFF and thereby test
the SFC availability. Alternately, the initiator can set the TTL to
other value to get the response from specific SFs and there by test
partial SFC availability. Alternately, the initiator could send OAM
packets with sequentially incrementing the TTL in NSH header to trace
the SFP.
It could be observed that ICMP at its current stage may not be able
to perform all required SFC OAM functions, but as explained above, it
can be used for basic OAM functions.
6.4.2. Seamless BFD Applicability
[RFC5880] defines Bidirectional Forwarding Detection (BFD) mechanism
for fast failure detection. [RFC5881] and [RFC5884] defines the
applicability of BFD in IPv4, IPv6 and MPLS networks. [RFC7880]
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defines Seamless BFD (S-BFD), a simplified mechanism of using BFD.
[RFC7881] explains its applicability in IPv4, IPv6 and MPLS network.
S-BFD could be leveraged to perform SF or SFC availability. An
initiator could generate BFD control packet and set the "Your
Discriminator" value as last SFF in the control packet. Upon
receiving the control packet, last SFF will reply back with relevant
DIAG code. We could also use the TTL field in the NSH header to
perform partial SFC availability. For example, the initiator can set
the "Your Discriminator" value to the SF that is intended to be
tested and set the TTL field in NSH header in a way that it will be
expired on the relevant SF. How the initiator gets the Discriminator
value of the SF is outside the scope of this document.
6.4.3. In-Situ OAM
[I-D.brockners-proof-of-transit] defines a mechanism to perform proof
of transit to securely verify if a packet traversed the relevant path
or chain. While the mechanism is defined inband (i.e, it will be
included in data packets), it can be used to perform various SFC OAM
functions as well.
In-Situ OAM could be used with O bit set and perform SF availability,
SFC availability of performance measurement.
6.4.4. SFC Traceroute
[I-D.penno-sfc-trace] defines a protocol that checks for path
liveliness and trace the service hops in any SFP. Section 3 of
[I-D.penno-sfc-trace] defines the SFC trace packet format while
section 4 and 5 of [I-D.penno-sfc-trace] defines the behavior of SF
and SFF respectively.
An initiator can control the SIL in SFC trace packet to perform SF
and SFC availability test.
6.5. Security Considerations
SFC and SF OAM must provide mechanisms for:
o Preventing usage of OAM channel for DDOS attacks.
o OAM packets meant for a given SFC should not get leaked beyond
that SFC.
o Prevent OAM packets to leak the information of an SFC beyond its
administrative domain.
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